Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/324

 312 FLIGHT pared to a compass set npon gimbals, where the universality of motion in one direction ensures comparative fixedness in another.&quot; &quot;All wings obtain their leverage by presenting oblique surfaces to the air, the degree of obliquity gradually ^increasing in a direction from behind, forwards and down wards, during extension when the sudden or effective stroke is being given, and gradually decreasing in an opposite direction during flexion, or when the whiff is being more slowly recovered preparatory to making a second stroke. The effective stroke in insects, and this holds true also of bird, is therefore delivered downwards and forwards, and not, as the majority of writers believe, vertically, or even slightly backwards. The wing in the insect is more flattened than in the bird ; and advantage is taken on some occasions of this circum stance, particularly in heavy-bodied, small-winged, quick-flying insects, to reverse the pinion more or less completely during the doicn and up strokes.&quot; &quot;This is effected in the following manner. The posterior margin of the wing is made to rotate, during the down stroke, in a direction from above downwards and from behind forwards, the anterior margin travelling in an opposite direction and reciprocating. The wing may thus be said to attack the air by a screwing movement from above. During the up or return stroke, on the other hand, the posterior margin rotates in a direction from below up wards and from before backwards, so that by a similar but reverse screwing motion the pinion attacks the air from beneath.&quot; &quot;A jigurc-of-8, compressed laterally and placed obliquely with its long axis running from left to right of the spectator, represents the movements in question. The doivn and up strokes, as will be seen from this account, cross each other, the wing smiting the air during its descent from above, as in the bird and bat, and during its ascent from below as in the flying fish and boy s kite (fig. 12). FIG. 12 shows the figurc-of-8 made by the margins of the wing in extension (continuous line), and flexion (dotted line). As the tip of the wing is mid-way between its margins, a line between the continuous and dotted lines gives the tigure-of-8 made by the tip. The arrows indicate the reversal of the planes of the wing, and show how the clown and up strokes cross each oilier. (1 ettigrew, 1867) &quot;The figure-of-8 action of the wing explains how an insect or bird may fix itself in the air, the backward and forward reciprocating action of the pinion affording support, but no propulsion. In these instances the backward and forward strokes are made to counterbalance each other. Although the figure-of-8 represents with considerable fidelity the twisting of the wing upon its axis during extension and flexion, when the insect is playing its wings before an object, or still better when it is artificially fixed, it is otherwise when the down stroke is added and the insect is fairly on the wing and progressing rapidly. In this case the wing, in virtue of its being carried forward by the body in motion, describes an undulating or spiral course, as shown in fig 13.&quot; Fio. 13. AVave track mado by the wing in progressive flight, a, b. crests of the wnvo ; &amp;lt;, &amp;lt;/, c, up strokes ; x, x, down strokes; f, point corresponding to the. anterior margin of the wing, and forming a centre for the downward rotation of the wing (,&amp;lt;/); g, point corresponding to the posterior margin of the wing, and forming a centre for the upward rotation of the wing ( /, f). (Pettigrcw, 1867.) . . . &quot;The down and up strokes are compound movements the termination of the down stroke embracing the beginning of the ijp stroke, and the termination of the up stroke including the begin ning of the down stroke. This is necessary in order that the down and up strokes may glide into each other in such a manner as to prevent jerking and unnecessary retardation.&quot; a &quot;The wing of the bird, like that of the insect, is concavo-convex, and more or less twisted upon ztec//when extended, so that the anterior or thick margin of the pinion presents a different degree of curva ture to that of the posterior or thin margin. This twisting is in a great measure owing to the manner in which the bones of the wing are twisted upon themselves, and the spiral nature of their articu lar surfaces, the long axes of the joints always intersecting each other at right angles, and the bones of the elbow and wrist making a quarter of a turn or so during extension and the same amount during flexion. As a result of this disposition of the articular sur faces, the wing may be shot out or extended, and retracted or flexed in nearly the same plane, the bones composing the wing rotating on their axes during either movement (fig. 14). The secondary action, Extension (elbow). Flexion (wrist). Flexion (elbow). Extension (wrist). I m. 14. a, b, line along which the wing travels during extension and flexion. The arrows indicate the direction in which the wing is spread out in extension and closed or folded in flexion. (Pettigrew, 1SG7.) or the revolving of the component bones on their own axes, is of the greatest importance in the movements of the wing, as it communi cates to the hand and forearm, and consequently to the primary and secondary feathers which they bear, the precise angles necessary for flight. It in fact ensures that the wing, and the curtain or fringe of the wing which the primary and secondary feathers form, shall be screwed into and down upon the wind in extension, and unscrewed or withdrawn from the wind during flexion. The wing of the bird may therefore be compared to a huge gimlet or auger, the axis of the gimlet representing the bones of the wing, the flanges or spiral 1 This continuity of the down into the up stroke and the converse is greatly facilitated by the elastic ligaments at the root or in the substance of the wing. These assist in elevating, and, when necessary, in flexing and elevating it. They counteract in some measure what may be regarded as the dead vruight of the wing, and are especially useful in giving it continuous play. thread of the gimlet the primary and secondary feathers&quot; (figs. 15 and 16) &quot;From this description it will be evident that by the mere rota tion of the bones of the forearm and hand the maximum and mini mum of resistance is secured much in the same way that this object is attainel by the alternate dipping Fir. 1-&quot;,. Right wing of the Red-legged Partridge and feathering of an (I cnlij-rubr/i). Dorsal aspect as seen from above. oar.&quot; &quot;The (1 ettigrew, 187.) wing, both when at rest and when in motion, may not inaptly be compared to the blade of an ordinary screw propeller as employed in navigation. Thus the general outline of the wing corresponds closely with the out line of the propeller (figs. 11, 16, and 18), andthetrack described by the wing in space is twisted upon itself propeller fashion 2 (figs. 12,20,21,22,23). The rrrpir vclocitv with Flr &quot; lr&amp;gt; -~ R ht wi &quot;S of tllG Red-legged Partridge great velocity With ( 1&amp;gt;eniij . ,. uhra) Dorsal and ventral aspects as een Which the Wlllg is from behind; showing auger-like conformation of driven converts the wing. Compare with figs. 11 and 18. (1 ettigrew, impression or blur 18(i -) into what is equivalent to a solid for the time being, in the same way that the spokes of a wheel in violent motion, as is well under stood, more or less completely occupy the space contained within the riin or circumference of the wheel&quot; (figs. 9, 20, and 21). 2 &quot;The importance of the twisted configuration or screw-like form cannot be over-estimated. That this shape is intimately associated with flight is apparent from the fact that the rowing feathers of the wing of the bird are every one of them distinctly spiral in their nature; in fact, one entire rowing feather is equi valent morphologically and physiologically to one entire insect wing. In the wing of the martin, where the bones of the pinion are shoit. and in some respects rudimentary, the primary and secondary feathers are greatly developed, mid bunked up in such a manner that the wing as a whole presents the same curves as those displayed by the insect s wing, or by the wing of the eagle, where the bones, muscles, and feathers have attained a maximum development. The con formation of the wing is such that it presents a waved ap]&amp;gt;earMnce in every direction, the waves running longitudinally, transversely, and obliquely. The greater portion of the wing may consequently be removed without essentially altering either its former its (unctions. This is proved by making sections in various directions, and by finding that in some instances as much as two-third* of the wing may be lopped off without materially impairing the power of flight&quot; (Trans, liny. Soc. E&amp;lt;Iin., vol. xxvi. pp. 3 - o, 3- (J).